This invention relates generally to improvements in turbocharger compressor wheels and the like, and related compressor wheel assemblies designed for extended wheel fatigue life. More particularly, this invention relates to an improved centrifugal compressor wheel of the boreless hub type, wherein the compressor wheel is adapted for simplified assembly in a turbocharger or the like.
Centrifugal compressor wheels in general are well known in the art for use in turbochargers and the like, wherein the compressor wheel comprises an aerodynamically contoured array of impeller blades supported on a central wheel hub section which is mounted in turn onto a rotatable shaft for rotation therewith. In the context of a turbocharger, by way of example, the wheel hub section conventionally includes a central axial bore through which the shaft extends, and a threaded nut is fastened over the shaft at a nose end of the compressor wheel to hold the hub section tightly against a shaft shoulder or other diametrically enlarged structure rotatable with the shaft. The rotatable shaft thereby drives the compressor wheel in a direction such that the contoured blades axially draw in air for discharge radially outwardly at an elevated pressure into a volute-shaped chamber of a compressor housing. The pressurized air is then supplied from the housing to the air intake manifold of a combustion engine for mixture and combustion with fuel, all in a well-known manner.
In recent years, improvements in compressor technology and design have resulted in progressive increases in compressor efficiencies and flow ranges, together with more rapid transient response characteristics. For example, compressor wheels for turbochargers are known wherein the contoured impeller blades exhibit compound and highly complex curvatures designed for optimum operational efficiency and flow range. Such complex blade shape is most advantageously and economically obtained by a casting process wherein the wheel hub section and blades are integrally formed desirably from a lightweight material, such as aluminum or aluminum alloy, chosen for its relatively low rotational inertia consistent with rapid wheel accelerations and decelerations during transient engine operating conditions.
Cast compressor wheels of this general type, however, have exhibited a relatively short, finite fatigue life resulting in undesired occurrences of wheel fatigue failure during operation. More specifically, when the compressor wheel is rotated at very high speeds, the cast aluminum material is subjected to relatively high centrifugal loading in a tangential direction particularly near the center, wherein the hub region of the compressor wheel must support the rapidly rotating wheel mass by the action of radial and tangential stresses. The impact of these stresses is especially severe when the compressor wheel is operated at a relatively high speed, rapid speed cycle environment as encountered, for example, in many modern turbocharger applications. Unfortunately, as previously described, the hub region of the compressor wheel normally includes a major void in the form of the central bore for receiving the rotatable shaft, wherein this central bore acts as a major stress riser rendering the wheel highly susceptible to fatigue failure in the hub region. This fatigue problem is compounded by the presence of any metallurgical imperfections, such as dross, voids, and/or inclusions which occur inherently during the casting process and tend to congregate in the hub region of the compressor wheel.
Improvements in cast compressor wheels have been proposed in the form of a compressor wheel having a boreless hub section to eliminate the stress riser site provided by the conventional shaft bore. While such alternative wheel designs theoretically provide extended fatigue life, a variety of significant design problems have been encountered.
For example, such boreless hub compressor wheels normally include an internally threaded sleeve extending coaxially from the boreless hub for attachment to the threaded end of a rotatable shaft. This threaded sleeve is formed to avoid any internal bore within the centrifugal compressor wheel along the axial span between the nose end and a plane corresponding with the maximum diameter of the wheel. To avoid undesired axial extension of the turbocharger or the like, the threaded sleeve is fitted through an adjacent opening in a compressor backplate for direct attachment to the rotatable shaft, with one or more seal rings being carried about the sleeve for sealing engagement with the backplate. However, assembly of the various components, including threaded attachment of the sleeve to the shaft while seating one or more seal rings within the backplate opening, can be extremely difficult. Moreover, when the threaded sleeve is cast integrally with the compressor wheel from the same lightweight material, the sleeve possesses inadequate structural hardness to support the seal ring or rings without undue wear. Alternative designs have envisioned welded attachment of a separate sleeve formed from a suitable hard bearing material onto a boreless hub wheel, as disclosed in U.S. Pat. No. 4,705,463, but this concept encounters significant balancing problems and undesirably introduces new stress riser zones within the hub as a result of exposure to welding heat.
There exists, therefore, a significant need for further improvements in turbocharger compressor wheels and related wheel assemblies of the boreless hub type, wherein a boreless hub compressor wheel is adapted for facilitated assembly with a turbocharger shaft in combination with sealed passage through a compressor backplate opening. The present invention fulfills these needs and provides further related advantages.